1. Field of the Invention
The present invention relates in general to digital data apparatus and relates more particularly to a digital data synchronizer for synchronizing a digital data system with an incoming digital data bit stream having a psuedo random bit sequence which is preamble to the beginning of data.
2. Background Art
In many digital data systems which utilize a serial digital data stream, it is necessary to know positively when the beginning of data occurs in the serial bit stream. Thus, digital data utilized in magnetic tape recorders, magnetic or optical disk drives, data transmission links, or the like, must be synchronized to the digital data system so that the digital data bit stream may be properly interpreted (such as by decoding, deformatting, deinterleaving, or the like). A very robust method of identifying the start of data within a digital data bit stream and of detecting such start with high reliability, is to utilize a psuedo random sequence of data bits preceeding the start of data. The psuedo random bit sequence can be based on any one of a number of primitive polynomials. Thus, U.S. Pat. No. 4,290,139, issued Sep. 15, 1981, inventor D. M. Walsh discloses use of an adaptive equalizer for exact preamble synchronization by modifying the digital filter section to recognize predetermined sequences of signals in the received signal. The system is described as being used in modems which utilize data transmission rates of 4,800 bits per second or more. Such a system would be inappropriate for use in preamble synchronization of high speed digital data systems.
There is thus, a need for a digital data synchronizer which is simple and requires a small amount of components; which provides positive location information with a relatively small number of bits in a valid serial sequence; which can resynchronize if a bit slip occurs after synchronization; which will provide a start time based on a predetermined countdown even if no valid sequence is seen; which will accommodate loss of data in the preamble bit sequence due to drop outs; and which can operate effectively at high data rates.